Polar feedback receiver for modulator

ABSTRACT

This disclosure relates to linearization in polar modulators of wireless communication devices and associated methods, to attain linear amplification and high power efficiency during transmission.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Patent Application of U.S. patentapplication Ser. No. 12/327,687 filed Dec. 3, 2008 entitled “PolarFeedback Receiver for Modulator” in the name of Nick Shute, et al. andis hereby incorporated by reference.

BACKGROUND

A modulator, such as a polar modulator, in a wireless communicationsdevice may require spectral quality for modulation schemes. Typically,most wireless communications devices are based on constant-envelopemodulation schemes (i.e., phase or frequency modulated). An advantagewith constant-envelope modulation scheme provides that a final radiofrequency (RF) power amplifier in the polar modulator does not have tobe linear, and as a consequence, the final RF power amplifier can beoperated in the most power efficient region near saturation. However, adrawback with constant-envelope modulation scheme is the inefficient useof the RF spectrum, where data rate transmission for a given bandwidthis not maximized.

To utilize the RF spectrum efficiently, a varying envelope and varyingphase modulation scheme may be used. When a varying envelope modulationis applied to a power efficient nonlinear amplifier, distortion may begenerated by the nonlinear amplifier which may cause interference withadjacent channels. The distortion may also result in detection error ofthe information signal at the receiver end of the communication channel.For most applications, the distortion is to be avoided, and may requirea linear amplifier; however, linear amplifiers typically have low powerefficiency, making a linear amplifier unsuitable for varying envelopeand phase modulation scheme. To obtain linear amplification and highpower efficiency for transmission in the polar modulator, linearizationof a power efficient and nonlinear amplifier may be implemented.

Linearization may refer to a method of compensation or correction ofnon-linearity in a polar modulator component to maintain stability atthe output of the polar modulator. Linearization of the polar modulatormay require a feedback receiver component to couple the polar modulatoroutput into a signal path of the amplifier's input. The feedbackreceiver component may produce linear amplification and power efficiencyin the polar modulator.

The feedback receiver component may be used as a quadrature demodulatorand require additional circuitry (e.g., high frequency local oscillator,mixers, 90 degree shifters, etc.). Such additional circuitry may drawsignificant amounts of current in the polar modulator. Furthermore,delay sensitivity may further be included due to different delays insignal sources and local oscillator input used in the quadraturedemodulator. The delay sensitivity may result in degradation of thecircuitry at microwave frequencies due to sub nanosecond delay variationrequirements.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is described with reference to the accompanyingfigures. In the figures, the left-most digit(s) of a reference numberidentifies the figure in which the reference number first appears. Thesame numbers are used throughout the drawings to reference like featuresand components.

FIG. 1 is a block diagram of a polar modulator component.

FIG. 2 is a block diagram of a Radio Frequency (RF) component for polarmodulator.

FIG. 3 is a block diagram of a polar feedback receiver.

FIG. 4 is a block diagram illustrating a specific implementation of thepolar feedback receiver in order to extract and measure the distortiononly.

FIG. 5 is a flow chart illustrating a process for linearization of apolar modulator using polar feedback receiver.

DETAILED DESCRIPTION

This disclosure is directed towards techniques and methods of performinglinearization in the polar modulator to attain linear amplification andhigh power efficiency during transmission. Although a polar modulator isdescribed, it is contemplated that the techniques and methods may beapplied to other modulators. The linearization may be implementedthrough the use of a polar feedback receiver in the polar demodulator todirectly extract magnitude and phase baseband signal in modulated radiofrequency (RF) output. The polar feedback receiver avoids the use ofadditional circuitry (e.g., quadrature demodulator) which may includeadditional current consumption and delay sensitivity in the polarmodulator.

FIG. 1 illustrates a polar modulator component 100 that includes abaseband component 102 and a RF component 104. During transmission,baseband component 102 may encode a data signal; identify the datasignal's prior state before modulation; convert the inphase signal (I)and quadrature phase signal (Q) into polar form; and transmit the datasignal for modulation. Output from the baseband component 102 isreferred to as a baseband signal. During demodulation, basebandcomponent 102 may identify the prior state of the data signal followingdemodulation. The baseband component 102 decodes the demodulated datasignal to re-create the data signal. The prior state of the data signalmay include attributes of the data signal, such as amplitudes of I andQ, frequency, and phase amount.

During modulation, RF component 104 may combine the baseband signal withRF frequency carrier to produce a modulated RF frequency signal, amplifythe modulated RF frequency signal, and further filter the modulated RFfrequency signal before transmission. The RF frequency carrier may referto the frequency of oscillation in the RF component 104 when amodulating baseband signal is not present. During receiving, the RFcomponent 104 may receive the modulated RF frequency signal, amplify themodulated RF frequency signal, filter the modulated RF frequency signal,and demodulate the modulated RF frequency signal.

A signal from peripherals, camera, display etc. 106 may be received byInput/Output (I/O) component 108 for initial processing. The I/Ocomponent 108 may convert analog data signals into digital data signals,while the digital data signals are maintained in the same state (i.e.,remain digital). Furthermore, the I/O component 108 may process the datasignals to produce the amplitudes of I and Q.

The data signals 110 are received by a digital signal processor (DSP)112. The DSP 112 may use a filter to limit the bandwidth forming aspectrum of the equivalent low pass signal or baseband signal. The DSP112 may include a Coordinate Rotation Digital Computer (CORDIC)component to transform the amplitudes of I and Q of the baseband signalinto equivalent polar representations. The equivalent polarrepresentations may contain the phase and magnitude of the basebandsignal, where the magnitude of the baseband signal may also refer toamplitude of the baseband signal.

After transformation of the baseband signal into the equivalent polarform, the baseband signal from the DSP 112 may pass through digitalinterface 114. The digital interface 114 may provide concurrentbi-directional communications between the baseband component 102 and RFcomponent 104. The digital interface 114 may contain clock signals toprovide timing references for transmit and receive communicationsbetween baseband component 102 and RF component 104.

During transmission, the baseband signal from digital interface 114 isreceived by phase modulator/analog signal processing component 116. Thephase modulator/analog signal processing component 116 may include anoutput that contains the modulated RF signal by varying the phase andmagnitude of the RF carrier corresponding to the baseband signal to betransmitted. The phase modulator/analog signal processing component 116may further support the linearization mechanism for the polar modulator100. A modulated RF signal 118, which is the output of phasemodulator/analog signal processing component 116, is passed to anonlinear amplifier 120 for further amplification before transmission.The nonlinear amplifier 120 may include a relatively high powerefficient amplifier suitable for varying envelope and phase modulationscheme.

A modulated RF signal 122 is an output of the nonlinear amplifier 120.The modulated signal 122 may contain distortions which may causeinterference in the adjacent channels. The distortions may be caused byunexpected delay between the phase modulation signal and amplitudemodulation signal. The unexpected delay may result in the signals (e.g.,phase modulation and amplitude modulation) being applied to differentportions of the RF carrier. Furthermore, the amplitude modulation maycause inadvertent phase modulation due to undesired feedback betweenphase modulation components and amplitude modulation components. Thedistortions may not only cause interference to the adjacent channels,but may also affect reception at the receiving end. Therefore, thedistortions should be avoided in order to produce linear amplification,and for high power efficiency transmission.

In an implementation, linearization may be used to compensate for thedistortions at the polar modulator 100. The linearization may includecoupling of the polar modulator 100 output (i.e., signal 122) into apolar feedback receiver 124. The polar feedback receiver 124 may be usedfor polar demodulation of the loop back polar modulator 100 output inorder to directly extract the phase and magnitude of the modulated RFsignal. A linearization mechanism that includes the polar feedbackreceiver 124 may be made adaptive by extracting an error signal (e.g.,phase and magnitude distortion) coefficient at the output (i.e., signal122) of the polar modulator 100. The error signal coefficient mayinclude unexpected delay between the phase modulation signal andamplitude modulation signal which may result in distortions in the polarmodulator 100. An input path signal 126 to the nonlinear amplifier 120may be adjusted in accordance with the extracted error signalcoefficient (e.g., phase and magnitude distortion) of the polarmodulator 100 during real time operation, to effectively andcontinuously minimize distortion in the polar modulator 100 output. Theadjusted modulated RF signal may be transmitted through antenna 128.

FIG. 2 illustrates a RF component 104 using the polar feedback receiver124 for linearization of the polar modulator 100. After transformationof the amplitudes of I and Q into polar form in DSP 112 of the basebandcomponent 102, a baseband phase signal 200 and baseband magnitude signal202 is received by RF component 104. The phase signal component of thebaseband signal, may phase modulate the RF frequency carrier through theuse of a local oscillator, phase detector, charge pump, filter, voltagecontrolled oscillator (VCO), and a multi modulus divider (MMD). Thephase modulated RF signal may further undergo amplitude modulation bythe baseband magnitude signal 202 to produce the varying envelope andphase modulation scheme.

The modulated RF signal 122 is coupled from the output of the polarmodulator 100. The modulated RF signal (i.e., signal 122) may includethe phase and magnitude of the baseband signal, together with thedistortions, which may be demodulated by polar feedback receiver 124.The polar feedback receiver 124 may implement polar demodulation of thecoupled modulated RF signal by directly extracting the phase andmagnitude baseband signal together with the distortions. The polarfeedback receiver 124 may extract the phase and magnitude basebandsignal and the distortions to be used for measuring the error signalcoefficient at the output of the polar modulator 100. The error signalcoefficient may be used to compensate the phase and magnitudedistortions to attain linear amplification and high power efficiencyduring transmission.

The output of polar feedback receiver 124 may include the phase signalinformation plus distortion signal 204 and the magnitude signalinformation plus distortion signal 206. Both signal 204 and signal 206may enter algorithm component 208 which may be used to calculate errorsignal coefficient. The error signal coefficient may include thedifference in gain between the phase and magnitude input sources, andthe extracted phase and magnitude signal (including the distortions).The difference in gain may correspond to the calculated amount ofdistortion at the output of the polar modulator 100 which may becompensated to attain linear amplification and high power efficiencyduring transmission.

The output 210 of the algorithm component 208 may include the magnitudeerror signal coefficient, and is received by Look Up Table (LUT) 212 formagnitude compensation. The output or signal 214 from algorithm 208 mayinclude the phase error signal coefficient, and is received by LUT 216for phase compensation.

Both LUT 212 and LUT 216 may include a data structure used to replace aruntime computation with a simpler lookup operation. The LUT 212 maycontain magnitude gain factors, corresponding to the magnitude errorsignal coefficient in signal 210, which may be multiplied with thebaseband magnitude signal 202 to produce the compensated magnitudesignal. The LUT 216 may include phase gain factors, corresponding to thephase error signal coefficient in signal 214, which may be multipliedwith the baseband phase signal 200 to produce the compensated phasesignal.

A compensated magnitude signal 218 may be used for amplitude modulationin the polar modulator 100. The compensated magnitude signal 218 mayfurther be used as an input signal into the polar feedback receiver 124in order to produce a limited modulated RF signal. The limited modulatedRF signal may include the modulated RF signal whose amplitude modulationcontent is removed. The amplitude modulation content may be removedthrough combination of the modulated RF signal with a low frequencyinverse baseband magnitude signal to produce the limited modulated RFsignal. The amplitude modulation content removal may contain a lowfrequency local oscillator signal that may be used to split themodulated RF signal into phase modulation and amplitude modulationcomponents. The local oscillator signal is the frequency signal that isnormally used to split the modulated RF signal into phase modulation andamplitude modulation components in a quadrature demodulator. However,delay sensitivity may occur due to different processing circuitry of themodulated RF signal and the local oscillator signal which may beincluded in the quadrature demodulator.

The low frequency inverse baseband magnitude may result from reversingthe compensated magnitude signal 218 of the polar modulator 100. Thecombined output of the modulated RF signal and the low frequency inversebaseband magnitude may include a limited modulated RF signal whose phasesignal can be directly extracted through the use of a phasediscriminator. The phase discriminator is a component where the phasesignal information is extracted for a given limited modulated RF signal.The limited modulated RF signal may further be used to extract themagnitude of the modulated RF signal together with the distortion due tononlinearities in the polar modulator 100.

The compensated magnitude signal 218 is received by a digital to analogconverter (DAC) 220 for amplitude modulation of the polar modulator 100.The DAC 220 may convert the compensated magnitude signal into analogcompensated magnitude signal 222 received by mixer component 224. Theanalog compensated magnitude signal 222 may be used to amplitudemodulate the phase modulated signal in mixer component 224. The mixercomponent 224 may combine the phase modulated RF signal with the analogcompensated magnitude signal 222 in order to provide the varyingenvelope and phase modulation scheme for the polar modulator 100.

The compensated phase signal 226 may be used as a control signal formulti-modulus divider (MMD) 228. The MMD 228 may be used in the polarmodulator 100 for low power, high operating frequencies, and high speedfrequency synthesis applications to cover multiple frequency bands. Thefrequency synthesis applications may refer to a phase locked loop (PLL)based frequency synthesizer where the MMD 228 is placed between theoutput, and the feedback input includes the ability of the PLL togenerate multiple frequencies at high speed applications. The MMD 228may divide the output frequency of VCO 230, where the output frequencyis received by MMD 228 as signal 232. The VCO 230 provides theup-converted frequency that is phase modulated by the baseband phasesignal.

Signal 234 is the output of MMD 228 received by phase detector (PD) 236.The PD 236 provides a control signal which corresponds to the differencebetween the output of MMD 228 and signal 238. The signal 238 may includea reference frequency generated by local oscillator 240, which producesa constant reference frequency used for phase modulation in the polarmodulator 100. The output or signal 242 of the PD 236 is received bycharge pump/filter 244. The charge pump/filter 244 may include apositive output current and a negative output current activated by thecontrol signal from PD 236. The positive output current or negativeoutput current may be filtered by a low pass filter to produce a DCvoltage or control voltage signal 246 to sustain operation of the VCO230 at a desired frequency. Signal 248 may include the phase modulatedoutput of VCO 230 entering into mixer component 224 for amplitudemodulation. The output of mixer component 228 which may contain thevarying envelope and phase modulated RF signal may pass through signal118 for amplification in the nonlinear amplifier 120.

FIG. 3 illustrates a polar feedback receiver 124 used for linearizationof the polar modulator 100. A compensated magnitude signal 218 isreceived by an inverse baseband magnitude component 300. The inversebaseband magnitude component 300 may include a gain that reverses thecompensated magnitude signal in signal 218. The reversed basebandmagnitude signal 302 may include a low frequency inverse basebandmagnitude signal used to directly extract the phase and magnitudesignals in the modulated RF signal. Signal 302 may include low frequencyinverse baseband magnitude received by DAC 304. The DAC 304 may convertthe digital low frequency inverse baseband magnitude into analog lowfrequency inverse baseband magnitude to match the analog modulated RFoutput in signal 122.

The analog signal low frequency inverse baseband magnitude signal 306 isreceived by mixer component 308 for amplitude modulation contentremoval. The mixer component 308 may operate as a variable gain stagewhere one input signal may include the modulated RF signal 122, and theother signal may include the analog low frequency inverse basebandmagnitude signal 306. The mixer component 308, operating as a variablegain stage, may strip away the amplitude modulation content withoutusing additional circuitry like a local oscillator in quadraturedemodulator. Removal of the amplitude modulation content in mixercomponent 308 may result in a limited modulated RF signal, and the phaseinformation may be restored efficiently as compared to using a typicalquadrature demodulator.

Limited modulated RF signal 310 may include a constant envelope phasemodulated signal. The limited modulated RF signal 310 may be used todirectly extract the phase component plus distortion of the modulated RFsignal 122 through the use of a phase discriminator 312. The phasediscriminator 312 may include amplitude variations in the output, whichis a function of phase variation in the limited modulated RF signal 310.The limited modulated RF signal 310 may also be used to extract themagnitude of the modulated RF signal 122 through mixer component 312.The mixer component 312 may combine the magnitude of the modulated RFsignal 122, and the limited modulated RF signal 310. The mixer component312 may include an output that includes the magnitude of the basebandsignal plus the distortion.

FIG. 4 illustrates an implementation of a polar feedback receiver 124,which includes direct extraction of distortions without the phase ormagnitude component. To extract the phase distortion only in the polarmodulator 100, the phase discriminator 312 may be implemented using asigma delta converter, MMD component, and time to digital converter(TDC) component. An MMD 400 component divides the limited modulated RFsignal 310, and removes the phase modulation, without the phasedistortion, through the use of a sigma to delta converter 402. The sigmato delta converter 402 may include high precision conversion of the lowfrequency baseband phase signal 226 into an analog baseband phase signal404. The output signal 406 of MMD 400 may include the analog phasedistortion output and is received by TDC 408. The TDC 408 compares thesignal edges of signal 406 to generate a delta time signal which can beconverted to a delta time phase signal. The output signal 410 of TDC 408may include only phase distortion.

In an implementation, the mixer component 314 may operate in linear mode(i.e., acts as a multiplier), and the signal 412 may include themagnitude of the modulated RF signal 122 and the square of thedistortion. An analog to digital converter (ADC) 414 converts theextracted magnitude signal together with the square of the distortion,into a digital signal 416. The digital signal 416 is received by 1/MSquare Root component 418. The 1/M Square Root component 418 divides thesignal 416 by the baseband magnitude signal and calculates the squareroot of the distortion signal to produce signal 420 which may onlyinclude the amount of magnitude distortion.

FIG. 5 illustrates an exemplary method 500 for feedback receiver usedfor linearization in a modulator. In an implementation, the exemplarymethod 500 can be implemented in the polar modulator 100. The exemplarymethod 500 is described with reference to FIGS. 1-4. The order in whichthe method is described is not intended to be construed as a limitation,and any number of the described method blocks can be combined in anyorder to implement the method, or alternate method. Additionally,individual blocks may be deleted from the method without departing fromthe spirit and scope of the subject matter described herein.Furthermore, the method can be implemented in any suitable hardware,software, firmware, or a combination thereof, without departing from thescope of the invention.

At block 502, inverting a magnitude of a baseband signal is performed.For example, inverse baseband magnitude component 300 may include a gainthat reverses the magnitude of the baseband signal to produce an inversemagnitude of the baseband signal in the polar modulator (e.g., polarmodulator 100).

At block 504, converting the inverse magnitude of the baseband signalinto analog form. In an implementation, a digital to analog convertercomponent (e.g., DAC 304) may convert the inverse magnitude of thebaseband signal into analog inverse magnitude of the baseband signal.

At block 506, removing an amplitude modulation content of a modulated RFsignal is performed. A mixer (e.g., mixer component 308), operating as avariable gain stage, may combine the analog inverse magnitude of thebaseband signal with the modulated RF signal to remove the amplitudemodulation content of the modulated RF signal. In an implementation, themixer (e.g., mixer component 308) contains an output that is referred toas a limited modulated RF signal.

At block 506, extracting a phase signal and distortion from the limitedmodulated RF signal is performed. A phase discriminator (e.g., phasediscriminator 312) may directly extract the phase signal and distortionfrom the limited modulated RF signal.

At block 508, extracting a magnitude signal and distortion is performed.The limited modulated RF signal may be used as an input to a mixer(e.g., mixer component 314) which directly extracts the magnitude anddistortion of the modulated RF signal by combining the modulated RFsignal with the limited modulated RF signal.

At block 510, calculating an error signal coefficient for linearizationof the polar modulator is performed. The algorithm (e.g., algorithm 208)may provide the error signal coefficient by calculating a differencebetween an input baseband signal sources (phase and magnitude) and theextracted signals (phase and magnitude signal with distortions) from themodulated RF signal.

CONCLUSION

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described. Rather,the specific features and acts are disclosed as preferred forms ofimplementing the claims. For example, the different circuits andcomponents may be configured to perform linearization in a polarmodulator.

What is claimed is:
 1. A feedback receiver comprising: an inversebaseband magnitude component configured to generate an inversecompensated magnitude signal from a compensated magnitude signal; afirst mixer component configured to combine the inverse compensatedmagnitude signal with a modulated radio frequency (RF) signal togenerate a limited modulated RF signal; and a second mixer componentconfigured to extract a magnitude signal plus distortion from thelimited modulated RF signal.
 2. The feedback receiver of claim 1,wherein the inverse baseband magnitude component includes a gain toreverse a low frequency baseband magnitude signal.
 3. The feedbackreceiver of claim 1, wherein the first mixer is configured to operate asa variable gain stage to generate a limited modulated RF signal enteringinto a phase discriminator, wherein the phase discriminator directlyextracts a phase signal plus distortion from the limited modulated RFsignal.
 4. The feedback receiver of claim 1, further comprising a multimodulus divider (MMD) and a sigma delta converter configured to extractthe phase distortion only from the limited modulated RF signal, whereinthe MMD divides the limited modulated RF signal and the sigma deltaconverter removes the phase modulation.
 5. The feedback receiver ofclaim 1, further comprising an analog to digital converter (ADC)component and 1/M square root component, wherein the magnitude signalplus distortion includes a magnitude signal plus a square of adistortion, and the magnitude signal plus distortion is converted intodigital form by the ADC component, and transformed by the 1/M squareroot component to obtain a magnitude distortion only.
 6. The feedbackreceiver of claim 1, further comprising a phase discriminator configuredto extract a phase signal plus distortion from the limited modulated RFsignal; and a mixer component configured to extract a magnitude signalplus distortion from the limited modulated RF signal.
 7. A method ofimplementing a feedback receiver for linearization of a modulatorcomprising: inverting a magnitude of a baseband signal to an inversemagnitude of the baseband signal; and removing an amplitude modulationcontent of a modulated RF signal by combining the inverse magnitude ofthe baseband signal with the modulated RF signal to produce a limitedmodulated RF signal.
 8. The method of claim 7, wherein the inverting themagnitude of the baseband signal includes the use of a compensatedmagnitude signal.
 9. The method of claim 7, wherein the removing of theamplitude modulation content of the modulated RF signal is generated bya mixer component operating in a variable gain stage to produce thelimited modulated RF signal.
 10. The method of claim 7, furthercomprising a multi modulus divider (MMD) and a sigma delta converter toextract the phase distortion only, wherein the MMD divides the limitedmodulated RF signal and removes the phase modulation through the use ofa sigma to delta converter.
 11. The method of claim 7, furthercomprising extracting a magnitude signal plus distortion from thelimited modulated RF signal, where the magnitude signal plus distortionincludes a square of the distortion, converting the magnitude signalplus distortion into digital form by an ADC component, and transformingthe digital magnitude signal plus distortion to obtain a magnitudedistortion only.
 12. The method of claim 7, further comprisingconverting the inverse magnitude of the baseband signal into analog formprior to removing the amplitude modulation content.
 13. The method ofclaim 7, further comprising extracting a phase signal plus distortionfrom the limited modulated RF signal.
 14. The method of claim 7, furthercomprising extracting a magnitude signal plus distortion from acombination of the limited modulated RF signal and the modulated RFsignal.
 15. The method of claim 14, further comprising calculating anerror signal coefficient for linearization of the modulator at leastpartially from the magnitude signal plus distortion.